Nowadays, petroleum-based polymers are widely used as traditional plastics in, for example, packaging and other consumables. These products, however, have various disadvantages, in particular, the accumulation of non-biodegradable plastics in the environment and the use of non-renewable raw materials. For this reason, during recent years, there is a growing interest in so-called biodegradable polymers as alternative solution for the traditional petroleum-based polymers. Biodegradable polymers are polymers obtained from molecules of vegetable origin.
These biodegradable polymers shall be referred to, hereinafter, as biopolymers.
Among such biopolymers, the importance of polylactic acid is steadily growing. One of the driving forces of this invention is the fact that the production cost of L-lactic acid has been substantially reduced by high-volume production of crops such as corn, grains and potatoes . . . . Plastics or resins such as polylactic acid manufactured on the basis of these natural raw materials are characterized by a high strength and good transparency.
A drawback of polylactic acid for use as plastic in industrial applications is, however, the low impact resistance, as well as the brittleness and resulting lack of flexibility. These material features are caused, among others, by a high crystallinity and a rigid molecular structure of this polymer. Nevertheless, amorphous formulations of polylactic acid are also available; these, however, are equally brittle and hard. This disadvantage limits its use in a great number of applications, in particular, for use in film or packaging material on a large scale.
It is known in the art to compensate for this drawback by softening polylactic plastics or resins by incorporation of plasticisers, by applying co-polymerization, or by blending polylactic acid with more soft polymers.
The use of plasticisers in resins to increase their flexibility is a well-known method, and is not particularly limited to biopolymers. By the use of plasticisers the possibilities and applications for these polymers are substantially increased. Plasticisers are usually available in liquid form and can be used to process resins in various technical processes, such as injection moulding, thermoforming, blown film and cast film extrusion, rotational moulding, fibre spinning, filament processing. The plasticisers can be optimized for use in various polymers. More in particular, the polarity of a plasticiser can match the polarity of the polymer or polymer composition, so as to obtain an efficient interaction between these components, which results in a high plasticizing efficiency and a low migration of the plasticiser. Plasticisers are used in various polymers, among which the most important are: polyvinylchloride, polyamide, polar rubbers, polyurethane, and also biopolymers like polylactic acid.
As described in European patent EP 2 202 267 B1, filed by Daihachi Chemical Industry Co., Osaka, Japan, published Dec. 7, 2011, a known disadvantage of adding plasticisers is their tendency to migrate to the surface of the plastic. Various disadvantages result therefrom: the colour and the surface appearance is modified, the transparency of the plastic is reduced, and the fragility and brittleness of the plastic increase over time due to reduction of the plasticizing effect by the migration of the plasticiser from the bulk of the plastic to the surface (see e.g. paragraphs 4 and 5 of the text). This patent describes the use of mixed esters of a.m. succinic acid to minimize the migration from the PLA-polymer. The ester form of this patent, however, is not mentioned, contrary to other symmetric esters, such as butyldiglycol adipate. The properties of the latter compound, however, are less beneficial.
The scientific article published in SEI Technical Review, Number 66, April 2008, pages 50-54 entitled “Development of Elastic Polylactic Acid material Using Electron Beam Radiation”, by Shinichi Kanazawa, describes the crystalline behaviour of polylactic acid and the ‘bleeding out’ of a plasticiser added to this compound. It confirms that, on the longer term, the polylactic acid based resin becomes brittle and hard.
The article does not specify plasticisers used. It discloses an electron-beam method to counter such bleeding-out phenomenon. Usually 10 to 30% by weight of the plasticiser should be added to the plastic so as to sufficiently reduce the glass transition temperature, usually to about room temperature.
Various plasticisers have been proposed in the state of the art to deal with this problem.
Japanese patent application No. 2000-198908, for example, discloses the use of acetyl tributyl citrate as plasticiser in polylactic acid.
In U.S. Pat. No. 8,232,354 B2, filed by Kao Corp. Tokyo, Japan, a method is described for the manufacture of plastic compounds on the basis of polylactic acid, wherein a polycarbodi-imide cross-linker has been added. The results of this compound in terms of plasticizing effects, however, were unsatisfactory.
U.S. Pat. No. 7,842,761, in the name of Lapol LLC, Santa Barbara, Calif., USA, describes a biological plasticiser for biopolymers such as polylactic acid, comprising a polyester plasticizing unit.
Column 1, lines 52 and following disclose the three basic techniques for plasticizing polymers of the polylactic acid type: addition of a plasticiser, co-polymerization and blending of flexible polymers.
More in particular, in this text, the drawbacks of the first two techniques are described.
U.S. Pat. No. 8,158,731 in the name of Hallstar Innovations Corp., Chicago, USA describes polymer blends comprising on the one part a biopolymer and on the other part an aliphatic polyester. The polyester is derived from repeating units of a dicarboxylic acid and an aliphatic diol.
As biopolymer, polylactic acid has been mentioned, for example on column 1, line 41. As dicarboxylic acids, for example, succinic acid and adipic acid have been mentioned (column 2, lines 13-14).
In the international patent application published as WO 2013/148255 in the name of 3M Innovative Properties Company, Saint Paul, Minn., USA, all claims are directed to citrate esters, comprising (amongst others) tetrahydrofurfuryl groups and a hydrogen or acyl group.
Reference is made e.g. to claim 13.
These plasticisers have been developed for use in ‘suitable polymeric materials’, see e.g. page 8, line 31, specifically mentioning polylactic acid. On page 8 the inventors extensively describe polylactic acid and on page 10 some commercial suppliers of this compound are set forth.
Page 7, lines 26-28 disclose that as well the citric acid as the tetrahydrofurfuryl alcohol may be produced by renewable raw materials. References to the preparation method for tetrahydrofurfuryl are set forth in the following lines.
Page 8 lines 20 and following describe the requirement of compatibility of the plasticiser with the polymer to be softened.
A suggestion is being made to the fact that the solubility nature of both compounds should be close to each other for a plasticiser to continue fulfilling its plasticizing function in the polymer.
Tri(alkyl)citrate has been mentioned on page 8, line 29.
Page 14, lines 23-29 describe the migration issue of the more traditional plasticisers when used in polylactic acid, and the fact that over time polylactic acid becomes brittle by the migration of the traditional plasticisers to the surface of the material (poor age stability).
In order to solve the problem of the migration of the plasticiser from the bulk of the polymer to the surface, a mixture could be used comprising plasticisers with quite different chemical structures. In such a case, however, other drawbacks appear: for example difficulties related to an appropriate and homogeneous mixing of these compounds in the biodegradable plastic, or their inherent incompatibility with the biopolymer.
The plasticisers known to be used in polymers such as polyvinylchloride do not necessarily act as plasticisers in polylactic acid in an acceptable manner: a minimal compatibility should be present between the plasticiser and the polymer to be plasticized. For this purpose, there should be a match between the chemical structure of the plasticiser and the polymer.